1. Field of the Invention
The present invention relates to an amorphous silicon deposition, and more particularly, to an amorphous silicon deposition method that is useful for sequential lateral solidification (SLS).
2. Discussion of the Related Art
Flat panel display devices that are thin, lightweight, and have low power consumption are in high demand. Flat panel display devices may be classified into two basic types. One type is a light-emitting display device that emits light to display images, and the other is a light-receiving display device that uses external light source to display images. Plasma display panels (PDPs), field emission display (FED) devices, and electro luminescence display devices are examples of light-emitting displays. Liquid crystal displays are examples of light-receiving displays. The liquid crystal display is widely used for laptop computers and desktop monitors because of its superior resolution, color range, and image quality.
Liquid crystal display (LCD) device uses the optical anisotropy and polarization properties of liquid crystal molecules to produce images. Liquid crystal molecules have a definite orientational alignment as a result of their long, thin shapes. That orientational alignment can be controlled by an applied electric field. In other words, as an applied electric field changes, so does the alignment of the liquid crystal molecules. Due to the optical anisotropy, the refraction of incident light depends on the orientational alignment of the liquid crystal molecules. Thus, by properly controlling an applied electric field, a desired light image can be produced.
While various types of liquid crystal display devices are known, active matrix liquid crystal display (AM-LCD) having thin film transistors (TFTs) and pixel electrodes arranged in a matrix are probably the most common. This is because an active matrix LCD can produce high quality images at reasonable cost.
The TFTs of LCDs generally include polycrystalline silicon (p-Si) or amorphous silicon (a-Si) as an active layer. Since amorphous silicon (a-Si) can be deposited at a low temperature to form a thin film on a glass substrate, it is more widely used for switching elements in liquid crystal display (LCD) devices. However, amorphous silicon (a-Si) has problems when used in large-area LCD devices because the electrical characteristics of amorphous silicon are problematic. In contrast to amorphous silicon, polycrystalline silicon provides faster display response times when used in TFT switching elements. Thus, polycrystalline silicon (p-Si) is better suited to large-area LCD devices, such as large laptop computers and television sets, which require greater field effect mobility.
During the formation of polycrystalline silicon for LCD applications, laser treatment techniques are frequently used. Such polycrystalline silicon also can be used in the driving circuits for the TFT switching devices.
FIG. 1 is a schematic diagram showing a main component of an active matrix liquid crystal display. In FIG. 1, the LCD device includes driving circuits 3 and an active matrix 4 on a substrate 2. The active matrix 4 is located in the central portion of the substrate 2, while the gate driving circuit 3a and the data driving circuit 3b are respectively located in the left and top portions of the substrate 2. In the active matrix 4, a plurality of gate lines 6 are disposed in a transverse direction, and a plurality of data line 8 are disposed in a longitudinal direction (perpendicular to the gate lines 6). Pairs of gate and data lines 6 and 8 define a plurality of pixel regions. The gate driving circuit 3a transfers address signals to the gate lines 6, and the data driving circuit 3b transfers display signals to the data lines in synchronism with the address signals. A plurality of electrically independent pixel electrodes 10 are arranged in pixel regions. At intersections of gate lines 6 and data lines 8 are a plurality of thin film transistors (TFTs) “T” that act as a switching devices. The TFTs form a matrix format, and each TFT “T” corresponds to a particular pixel electrode 10.
The gate driving circuit 3a and the data driving circuit 3b are electrically connected to a control signal input port 12 that is interconnected to a controller (not shown) that controls the gate driving circuit 3a and the data driving circuit 3b. The gate driving circuit 3a and the data driving circuit 3b include CMOS (complementary metal oxide semiconductor) transistors that act as inverters that transfer signals to the pixel electrodes 10. Namely, the gate driving circuit 3a and the data driving circuit 3b are silicon thin film complementary metal oxide semiconductor (CMOS) structures having a shift register. The shift register can be a static or dynamic circuit of complementary (P type and N type) or monoconductive TFTs.
It is very important that an active matrix liquid crystal display (AM-LCD) device similar to FIG. 1 have an active matrix 4 and driving circuits 3 with low off state leakage currents. Additionally, it is important that the CMOS transistors have high field effect mobility. However, polycrystalline silicon used in TFT switches and CMOS transistors have many crystal grains, and thus grain boundaries. Those grains and boundaries interrupt carrier movement and cause deterioration of the active matrix elements. If the grains are larger and the grain boundaries more regularly distributed within the polycrystalline silicon, the field effect mobility increases. Thus, a silicon crystallization method that produces large grains is an important issue.
To solve the problems mentioned above, the crystal field distribution must be controlled, or a single crystal device is required. A technique of forming a single crystalline silicon layer on a glass substrate by sequential lateral solidification (SLS) is described in Robert S. Sposilli, M. A. Crowder, and James S. Im, Mat. Res. Soc. Symp. Proc. Vol. 452, 956–957, 1997. That technique uses the fact that silicon grains tend to grow laterally from the interface between liquid and solid silicon. Based on that principles the reference teaches crystallizing amorphous silicon layer by controlling the laser energy and irradiation range of a moving laser beam to produce silicon grains having predetermined length, as shown in the following example. Accordingly, sequential lateral solidification (SLS), which induces lateral growth of silicon grains, can form single-crystal silicon film using laser energy.
FIG. 2 is a graph illustrating grain size verses laser energy density, and FIGS. 3A–3C present cross-sectional views for explaining the mechanism of forming p-Si film composed of large grains. As shown in FIGS. 3A to 3C, a buffer layer 12 and an amorphous silicon layer 14 are sequentially formed on a transparent substrate 1.
Referring now to FIGS. 2 and 3A, the first regions is a partial melted regime. When a laser beam having an energy density within the first region irradiates the amorphous silicon layer 14, only a surface portion “A” of the amorphous silicon layer 14 is melted. Thereafter, during an annealing process a plurality of small grains “G1” are formed in a vertical direction.
The second region of FIG. 2 illustrates a laser beam having an energy density that nearly melts the complete region. When a laser beam having an energy density in the second region irradiates the amorphous silicon layer 14, almost all of the amorphous silicon is melted, reference FIG. 3B. Furthermore, a plurality of seeds 13 is formed between the amorphous silicon layer 14 and the buffer layer 12. Due to the seeds 13, the silicon grains tend to grow horizontally. However, since the plurality of seeds 13 are distributed randomly over the transparent substrate 1, it is difficult to obtain a plurality of uniform grains “G2” even though the grains “G2” are larger.
The third region of FIG. 2 illustrates a complete melting regime. When a laser beam having an energy density within the third region irradiates the amorphous silicon layer 14, all of the amorphous silicon is melted, reference FIG. 3C. Then, homogeneous nucleation is conducted during the annealing process. Therefore, a plurality of nuclei 15 is formed in the melted silicon, and fine grains “G3” are obtained.
In view of above-mentioned silicon crystallization mechanism, suitable laser energy densities can be obtained experimentally, and the region of suitable laser energy densities can be referred as a process window.
FIG. 4 is schematically illustrates sequential lateral solidification (SLS). When a first laser beam having an energy density that completely melts the amorphous silicon layer 20 a first complete melting region “I” is induced. Furthermore, a plurality of seeds 24 are formed at interfaces 24 between the solid phase amorphous silicon and the liquid phase silicon due to the liquid-solid phase contact. During annealing, the plurality of seeds 24 tend to grow laterally (opposing vertical directions in FIG. 4), thereby generating a plurality of first grains 26. Furthermore, during annealing a plurality of fine grains 28 can be formed near the middle of the first melting region “I”, and thus the first grains 26 divided that region into two parts.
After forming the first grains 26, a second laser beam having an energy density that completely melts the silicon layer is applied. The second laser beam beneficially has the same energy density and beam width as the first laser beam. Before the second laser beam irradiates the substrate 22, it is moved such that the bottom area of the first melting region “I” (portion II) and the fine grains 28 will be irradiated. The second laser irradiation completely melts the portion “II” a second time and re-melts the first grains. During annealing the first grains 26 of the top area “III” tend to grow laterally until it forms a single crystal grains 30. Namely, the single crystal grains 30 are formed by repeating the first and second laser beam irradiation and annealing processes.
In the above-mentioned SLS process the distance between the first and second laser beam irradiations is beneficially the length of lateral growth of the first grains 26 in the top area “III”. That length is often referred to as a translation distance. By way of adjusting the translation distance, the fine grains 28 must be removed from the middle of the first laser beam width “I”.
For example, a laser beam having a beam width of 1 to 3 millimeters can be used and an overlapping ratio of 90% is suitable. The amorphous silicon layer is usually formed with a thickness of 500 angstroms. If the thickness of the amorphous silicon layer was greater than 500 angstroms, the laser beam would have to a greater laser density energy to melt the thicker amorphous silicon, which would increase manufacturing costs and possibly slow down the fabrication process, with no improvement in the characteristics of the thin film transistors being produced. Accordingly, when depositing the amorphous silicon to induce the polycrystalline silicon, the amorphous silicon layer should maintain a thickness ranging from 500 to 600 angstroms.
Since SLS crystallization uses a laser beam having an energy density sufficient to completely melt the silicon layer, the thickness of the amorphous silicon layer is an important issue. If the amorphous silicon layer has a thickness of about 500 angstroms, defects occur during SLS crystallization. When the translation distance is short defects such as a pilling phenomenon and an agglomeration phenomenon may occur and the range of the process window decreases.
FIG. 5 illustrates laser energy density during SLS crystallization when the amorphous silicon layer has a thickness of 500 angstroms. In SLS crystallization, when the laser energy density is about 410 mJ/cm2 for the formation of single crystal silicon, the translation distance can range from 0.05 to 0.9 micrometers to compose the process window. Furthermore, when the laser energy density is about 450 mJ/cm2, the translation distance ranges from 0.6 to 1.0 micrometers.
If the translation distance is more than 0.95 micrometers at a laser energy density of 425 mJ/cm2, as shown in FIG. 5, resulting silicon becomes polycrystalline silicon instead of single crystalline silicon. On the other hand, if the translation distance is less than 0.3 micrometers at an laser energy density of 425 mJ/cm2, resulting silicon becomes crystalline silicon having dome defects, such as pilling and agglomeration. Accordingly, 500 angstroms thick amorphous silicon has a narrow process window and is susceptible to defects.